There is a need in the field of radio frequency (RF) technology to provide a filtering device that is adapted to reliably isolate input signals in closely-spaced frequency bands even when the filtering device is exposed to temperature changes. This is a requirement in modern telecommunication systems in which one needs to separate two or more input signals that are closely-spaced in frequency. The filtering device needs to have a pass band for a first radio signal in a first frequency band and a stop band for a second frequency in a second frequency band. The second frequency band is closely-spaced in frequency to the first frequency band.
In the prior art such filtering characteristics were achieved by filtering the input signal using a band-pass filter and forwarding the output of the band-pass filter to a band-stop filter such that the band-stop filter would suppress an unwanted frequency band closely-spaced to a wanted frequency band. As the band-pass filter is typically different in design from the band-stop filter, a temperature tracking of the band-stop filter may be substantially different. When filtering closely-spaced ones of the frequency bands with a cascade of the band-pass filter and the band-stop filter, it is of interest for a pass band of the band-pass filter and a pass band of the band-stop filter not to impinge on each other. If the pass band of the band-pass filter and the pass band of the band-stop filter are overlapping, i.e. impinging, the input signals could pass the combined band-pass filter and band-stop filter at one or more unwanted frequencies.
Another alternative is to cascade a first band-pass filter and a second band-pass filter. A 3 dB point of the first band-pass filter could coincide with a 3 dB point of the second band-pass filter and result in a 6 dB attenuation at frequencies where only 3 dB attenuation is desired or permitted. Likewise other points in the pass-bands of the first band-pass filter and the second first band-pass filter will result in an attenuation which is twice as great. This “double” attenuation could result in a marked increase in error vector magnitude (EVM) for the signals passing through the cascaded band pass and in consequence may exceed a threshold for the EVM in, for example, 3GPP standards.
Quite frequently the temperature tracking, i.e. a response to temperature changes, of the band-pass filter is different from the temperature tracking of a band-stop filter. Consequently a combination of the band-pass filter and the band-stop filter, as known in the prior art, may generate an overlapping of the pass band of the band-pass filter and the pass band of the band-stop filter when being exposed to the temperature changes. In a radio system the filtering elements are normally exposed to temperature changes. Such temperature changes may stem, for example, from an exposure to variable environmental conditions and/or heat generated in the radio system, but are not limited thereto. There is a need for the filtering device to be adapted to separate or isolate closely-spaced ones of the frequency bands when the filtering device is exposed to the temperature changes.
U.S. Pat. No. 5,473,295 to LK-Products OY Finland teaches a SAW filter being coupled to a receiver (Rx) branch of a duplex filter. The provision of the SAW filter increases the stop band attenuation of the duplex filter. The SAW filter is configured as a notch filter. The SAW filter improves the rejection of a band-pass filter in a mobile radio telephone.
Jiguo Wen, et al. disclose in “Suppression of Reflection Coefficients of Surface Acoustic Wave Filters using Quadrature Hybrids”, published in IEEE transactions on Ultrasonics, Ferroelectrics and Frequency Control, Volume 53, issue 10, pages 1912-1917, the use of a quadrature hybrid to improve the input and output matching characteristics of a pair of identical SAW filters. The Wen paper uses two quadrature hybrids.